Can I book a hotel room for 2 and then bring in a 3rd person?

If all three adults, they might deny three occupancy in one room. But if one kid and two adults, there shouldn't be much of a problem. They'll probably just charge extra for the kid's bed and breakfast.

When booking a hotel in Abu Dhabi, it's common for families to reserve a room with a king-size bed and a sofa bed. To accommodate a third guest, simply omit the guest count on the booking website and contact the hotel directly to confirm the arrangement. They will typically note the additional guest on the room key and charge a small extra fee, which is often more convenient than reserving a separate room.